This is a note to let you know that I've just added the patch titled

sysrq: Fix warning in sysrq generated crash.

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: sysrq-fix-warning-in-sysrq-generated-crash.patch and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 984cf355aeaa8f2eda3861b50d0e8d3e3f77e83b Mon Sep 17 00:00:00 2001

From: Ani Sinha ani@arista.com Date: Thu, 17 Dec 2015 17:15:10 -0800 Subject: sysrq: Fix warning in sysrq generated crash.

From: Ani Sinha ani@arista.com

commit 984cf355aeaa8f2eda3861b50d0e8d3e3f77e83b upstream.

Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") replaced spin_lock_irqsave() calls with rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not disable preemption, faulthandler_disabled() in __do_page_fault() in x86/fault.c returns false. When the code later calls might_sleep() in the pagefault handler, we get the following warning:

BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187 in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a

To fix this, we release the RCU read lock before we crash.

Tested this patch on linux 3.18 by booting off one of our boards.

Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")

Signed-off-by: Ani Sinha ani@arista.com Reviewed-by: Rik van Riel riel@redhat.com Signed-off-by: Paul E. McKenney paulmck@linux.vnet.ibm.com Signed-off-by: Davidlohr Bueso dbueso@suse.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- drivers/tty/sysrq.c | 6 ++++++ 1 file changed, 6 insertions(+)

--- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -133,6 +133,12 @@ static void sysrq_handle_crash(int key) { char *killer = NULL;

+ /* we need to release the RCU read lock here, + * otherwise we get an annoying + * 'BUG: sleeping function called from invalid context' + * complaint from the kernel before the panic. + */ + rcu_read_unlock(); panic_on_oops = 1; /* force panic */ wmb(); *killer = 1;

Patches currently in stable-queue which might be from ani@arista.com are

queue-4.4/sysrq-fix-warning-in-sysrq-generated-crash.patch